I had a cat years ago that developed an abscess every summer right there on the back near the tail. And it was from flea bites. We had to be very diligent about keeping the fleas under control. Not sure why that area was so reactive but it was a battle.
Itchy dermatological lesions can be caused by demodex, scabies, notodectes, or other mites, flea allergy dermatitis, ringworm, food allergies, and other problems. Chewing on fur can also be caused by behavioral barbarism, also known as psychogenic alopecia, but this is an uncommon cause.
The cat who does go out can bring in fleas unless that cat is protected with an effective flea and tick preparation, such as Advantage, Frontline, Vectra or other topical product. This cat, if unprotected could bring in a single flea. If the itchy cat has flea allergy dermatitis, a single flea could bite her and she could be itchy for a month.
It would be best to have this problem addressed by your veterinarian.